GMAT Critical Reasoning

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In a study conducted in Pennsylvania, servers in various restaurants wrote "Thank you" on randomly selected bills before presenting the bills to their customers. Tips on these bills were an average of three percentage points higher than tips on bills without the message. Therefore, if servers in Pennsylvania regularly wrote "Thank you" on restaurant bills, their average income from tips would be significantly higher than it otherwise would have been.

Which of the following is an assumption on which the argument relies?

A The "Thank you" messages would have the same impact on regular patrons of a restaurant as they would on occasional patrons of the same restaurant.

B Regularly seeing "Thank you" written on their bills would not lead restaurant patrons to revert to their earlier tipping habits.

C The written "Thank you" reminds restaurant patrons that tips constitute a significant part of the income of many food servers.

D The rate at which people tip food servers in Pennsylvania does not vary with how expensive a restaurant is.

E Virtually all patrons of the Pennsylvania restaurants in the study who were given a bill with "Thank you" written on it left a larger tip than they otherwise would have.